The Ebola outbreak in West Africa has taken an unprecedented toll on health care workers, infecting more than 240 and killing more than 120, the World Health Organization says.

"In many cases, medical staff are at risk because no protective equipment is available – not even gloves and face masks," the agency says in its latest update on the outbreak in Guinea, Liberia, Nigeria and Sierra Leone.

The compassionate instincts of those who sometimes rush to aid "visibly ill" people without pausing to protect themselves also put health workers at increased risk, the agency says. Staffs also are overworked, stretched thin and exhausted, which can contribute to mistakes in infection control, WHO says.

On Tuesday, WHO said it pulled workers from one of its own posts at Kailahun in Sierra Leone after a worker became infected. It said it will reopen the post after it reassesses safety there. The workers remain in the country, WHO said.

"This was the responsible thing to do," said a statement from the agency's Daniel Kertesz. The workers "are exhausted from many weeks of heroic work, helping patients infected with Ebola. When you add a stressor like this, the risk of accidents increases."

Among medical workers previously infected in the outbreak were two Americans, Nancy Writebol and Kent Brantly, who were transported from Liberia to Atlanta for care and released from Emory University Hospital last week.

Writebol, a clinic aide, and Brantly, a physician, recovered after treatment with an experimental medication called ZMapp. A Spaniard infected with Ebola also received the drug but died. In addition, a Liberian doctor, Abraham Borbor, died despite receiving ZMapp, Liberian Information Minister Lewis Brown said Monday. It is not possible to know from such anecdotal reports whether the drug helps or hurts, medical experts say.

Overall, the Ebola virus still has the "upper hand" in an outbreak that has sickened more than 2,600 and killed more than 1,400, says Tom Frieden, the director of the U.S. Centers for Disease Control and Prevention.

Frieden is visiting the outbreak area and spoke Monday in Liberia. He expressed optimism that the outbreak can be contained.

"Ebola doesn't spread by mysterious means, we know how it spreads," he said in remarks broadcast on Liberian TV. "So we have the means to stop it from spreading, but it requires tremendous attention to every detail."

The outbreak in West Africa is the largest ever and will cost $430 million to control, according to a draft WHO report obtained by Bloomberg news.

A possibly separate outbreak may be happening in the Democratic Republic of Congo. On Sunday, government Health Minister Felix Kabange Numbi said Ebola had killed 13 people there, including five health care workers. Initial tests showed a different strain of the virus than the one spreading in the other countries, he said.